Sahih Bukhari | Volume 3 | Hadith Number 220

Quraish used to fast on the day of 'Ashura' in the Pre-Islamic period, and Allah's Apostle too, used to fast on that day. When he came to Medina, he fasted on that day and ordered others to fast, too. Later when the fasting of the month of Ramadan was prescribed, he gave up fasting on the day of 'Ashura' and it became optional for one to fast on it or not.